Surface treatment of magnetic particles for use in reprographic processes

ABSTRACT

Provided is a method for producing nano-size magnetic particles, and particularly magnetite and maghemite particles, that are useful in preparing toner products for reprographic processes. The magnetic particles are made of a controlled size through the use of a microemulsion. Precursor particles are precipitated in droplets of a disperse aqueous phase of the microemulsion. The precursor particles are oxidized in a carefully controlled environment to form the desired magnetic particles and to avoid overoxidation to produce undesirable nonmagnetic particles, such as hematite. In one embodiment, the nano-size magnetic particles are treated to improve their hydrophobicity. The treated particles have a reduced tendency to agglomerate and are easier to disperse in the preparation of toner products. The hydrophobic treatment may include connecting hydrophobic chemical groups to the magnetic particles through the use of silane coupling agents. In addition to improving the flowability characteristics of the magnetic particles, hydrophobic treatment may also be used to at least partially mask the inherent color of the nano-size magnetic particles that could otherwise interfere with preparation of color toners for use in developing sharp color images.

FIELD OF THE INVENTION

The present invention involves the manufacture of nano-size magneticparticles useful for reprographic processes and the nano-size magneticparticles produced thereby.

BACKGROUND OF THE INVENTION

Reprographic processing involves the formation of printed images onsheet substrates such as paper. Examples of reprographic processinginclude the printing of facsimile transmissions, making of photocopies,and printing of electronically stored information from a computer.Technological developments over the last several years have madeavailable high quality, relatively low cost facsimile machines,photocopiers and printers for black print applications. The same lowcost, high quality options are not available, however, for color printapplications.

High quality color ink jet printers have been introduced in recentyears. These printers use a liquid jet spray to form a color image onpaper. Although print images are of high quality, the printing processis slow and requires special paper, which increases the cost of theprocess.

Color laser printers have also been introduced, in which a dry toner ismechanically applied to a drum to develop the desired image. These colorlaser printers, however, require image-on-image development, with eachcolor being developed separately to create the ultimate desiredcomposite color image. Image-on-image development requires that eachseparate color image be transferred for storage onto an intermediatedevice, where the composite image is developed by overlaying individualcolor images. The finished composite image is then transferred to apiece of paper for printing. Each color is thus developed in a separatestep, which significantly slows the printing operation and requires theexpense and complications associated with the intermediate storagedevice.

Fast and relatively inexpensive laser printers are available for blackprinting that do not require mechanical application of toner to a drum.These printers use "jump gap" technology in which magnetic particlesembedded in toner particles assist the toner particles in "jumping"across a gap and onto a drum where the image is developed. Such jump gapprocessing does not require mechanical application of the toner to thedrum. Color printing could, therefore, theoretically be accomplished onsuch a laser printer without the burden and expense of theimage-on-image development process of current color laser printers. Oneproblem with adapting the jump gap technology for color printing,however, is that the inherent color of magnetic particles used in thetoner significantly dulls and distorts the sharpness of pigments or dyesthat may be used in the toner to provide the desired color. One proposalfor reducing the dulling and distortion of color caused by magneticparticles is to use nanocrystalline gamma-Fe₂ O₃ as the magneticparticles (R. F. Ziolo et al., Matrix-Mediated Synthesis ofNanocrystalline gamma-Fe₂ O₃ : A New Optically Transparent MagneticMaterial, Science, volume 257, July 1992, pp. 219-223). Due to theirsmall size, it is proposed that such particles would tend to be moretransparent than the larger magnetic particles currently used in tonercompositions and would, therefore, not distort colors as much as thecurrently used magnetic particles.

The nanocrystalline gamma-Fe₂ O₃ reported by Ziolo et al. was formed inthe porous network of an ion-exchange resin. The resin was used toconstrain the reaction environment and to isolate and stabilize theparticles during their formation. The resulting composite is in the formof clear, amber colored resin beads having a diameter of about 50-100micrometers. These beads are much too large for use in reprographicprocesses. Also, the nanocrystalline gamma-Fe₂ O₃ is difficult toseparate from the resin in a manner to provide useful nano-sizedparticles that could be incorporated into a toner product.

It has also been proposed that nano-sized magnetite particles for use intoners could be prepared by microbial action (U.S. Pat. No. 4,886,752 byLovley, issued Dec. 12, 1989). Lovley discloses extracellular, microbialproduction of ultrafine-grained magnetite ranging in size from about 10to 5 nanometers. Such microbially produced magnetite could, however, bedifficult to separate and sufficiently clean for practical use inreprographic processes. Furthermore, Lovley reports that the nano-sizeparticles are present as aggregates. Such aggregates would not besuitable for use in toners and would require significant processing tobreak up the aggregates, which would be required to obtain the fullbenefit of the small size of individual grains. Moreover, the microbialprocess would not provide significant flexibility in preparing magneticparticles of different sizes for different reprographic applications.

Based on the foregoing, there is a need for improved nano-size magneticparticles for use in reprographic processing and for processes of makingsuch particles, especially for use in reprographic processes in whichcolor images are desired.

SUMMARY OF THE INVENTION

According to the present invention, nano-size magnetic particles, andparticularly magnetic particles of magnetite or maghemite, are providedfor use in toners for reprographic processes. During manufacture of thenano-size magnetic particles, the disperse phase of a microemulsion isused to constrain the size of particles being formed. In a water-in-oiltype of microemulsion, a metal-containing reactant is dissolved in thedisperse aqueous phase. The metal-containing reactant is reacted in thedisperse phase to form precursor particles of controlled size in thedesired nano-size range. The precursor particles may then be convertedto the desired magnetic material through use of a carefully controlledoxidation step. The oxidation must be carefully controlled to avoidexcessive oxidation that would result in formation of undesirablenonmagnetic material, such as hematite in the case of iron-containingmaterials. The process permits careful control of particle size.Particles produced preferably are smaller than about 60 nanometers andmore preferably smaller than about 40 nanometers. One important aspectof the present invention is that the magnetic particles, although beingin the nanometer size range, exhibit adequate magnetic properties topermit their use in toner products for reprographic processes. Also, themethod of manufacture of the present invention produces particles whichare of high purity, are clean and are easy to separate for use in tonerproducts.

Another aspect of the present invention is use of the nano-size magneticparticles in the production of toner products. The magnetic particlesare mixed with a polymer resin and, optionally, other additives asdesired. The mixture is formed into toner particles having a sizegenerally smaller than about 10 microns, and preferably smaller thanabout 8 microns. Because the toner particles comprise the nano-sizemagnetic particles, colors are not as detrimentally affected as withlarger, conventionally available magnetic particles.

In one embodiment of the present invention, the nano-size magneticparticles are subjected to a surface chemical treatment to improve thehydrophobicity of the magnetic particles and to thereby also improve theflowability of the particles. Such a surface treatment reduces thepotential for agglomeration of the nano-size particles and makes iteasier to disperse the nano-size particles in a polymer resin to make atoner product.

During the surface treatment, hydrophobic chemical groups are placedabout the surface of the magnetic particles. In one embodiment, thehydrophobic chemical groups are provided by fatty acid salts. Inanother, preferred embodiment, the hydrophobic chemical groups areconnected with the magnetic particles through covalent bonding. Thecovalent bonding preferably involves the use of a silane coupling agentintermediate between the magnetic particles and the hydrophobic chemicalgroups.

In another embodiment, the hydrophobic chemical groups at leastpartially mask the inherent color of the underlying magnetic particles.Also, the hydrophobic group could contain a chromophore to impart somecoloration to the magnetic particles to further reduce detrimentaleffects on the sharpness of colors when the magnetic particles areblended into a color toner product.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a microemulsion useful in the present invention.

FIG. 2 shows a reverse micelle of a microemulsion useful in the presentinvention.

FIG. 3 is a graph of the size of reverse micelles in a microemulsionuseful in the present invention as a function of the relativeconcentrations of water and a surfactant.

FIG. 4 is a flow diagram showing process flow for one embodiment of thepresent invention.

FIG. 5 shows chemical equations of reactions believed to be involvedwith one embodiment of the present invention for surface treatingnano-size magnetic particles.

FIG. 6 is a graph showing an x-ray diffraction pattern of nano-sizemagnetite particles prepared according to one embodiment of the presentinvention.

FIG. 7 is a graph showing an x-ray diffraction pattern of nano-sizemaghemite particles prepared according to one embodiment of the presentinvention.

FIG. 8 is a transmission electron micrograph of hydrophobically treatedmagnetite particles prepared according to one embodiment of the presentinvention.

DETAILED DESCRIPTION

The present invention involves nano-size magnetic particles useful inreprographic processes. As used herein, nano-size magnetic particlesinclude magnetic particles having a size of smaller than about 100nanometers. The present invention also involves methods for makingnano-size magnetic particles for use in reprographic processes, andespecially in non-impact reprographic processes for recording a printedimage on a sheet substrate, such as paper. Reprographic processesinclude photocopying, printing of facsimile transmissions and normalprinting operations controlled by a microprocessor. The magneticnanoparticles of the present invention are particularly useful forreprographic processes using jump gap technology, in which print medium,such as toner, is transferred to a drum to develop a desired imagewithout mechanical application of the print medium to the drum.Preferred magnetic particles of the present invention include magnetite(Fe₃ O₄) and maghemite (gamma-Fe₂ O₃), although the principles discussesherein with reference to magnetite and maghemite could apply equally toother nano-size magnetic particles as well.

Manufacture of the magnetic particles of the present invention may beaccomplished by using the droplets of a disperse phase of amicroemulsion as microreactors to limit the size of resulting particles.In particular, it is preferred that aqueous phase droplets of awater-in-oil microemulsion function as the microreactors. These disperseaqueous phase droplets are often referred to as reverse micelles, andthe microemulsion may be referred to as a reverse micellar system.

FIG. 1 shows a microemulsion having a continuous phase 102 of anon-polar organic liquid, and a disperse phase 104 of droplets of apolar liquid. The disperse phase typically comprises an aqueous liquid.At the interface between the continuous phase 102 and the disperse phase104 is a surfactant 106 that helps to stabilize the microemulsion.

FIG. 2 shows an expanded view of a reverse micelle of a water-in-oilemulsion having an aqueous core of disperse phase 104 surrounded bysurfactant molecules each having a hydrophilic head 108, in associationwith the aqueous liquid of the disperse phase, and a hydrophobic tail110, in association with the nonpolar organic liquid of the continuousphase 102.

Reverse micelles, as shown in FIG. 2, typically have a size of fromabout 1 nanometer to about 100 nanometers diameter, with the size beingcontrolled by the relative amount and type of surfactant used. FIG. 3shows a plot of Stokes radius (in angstroms) of reverse micelles as afunction of the ratio of water concentration to surfactantconcentration. In a system having aerosol OT (AOT) as a surfactant andisooctane as a continuous phase. As shown in FIG. 3, the size of thereverse micelles generally decreases with increasing relative amounts ofsurfactant. Preferred surfactants for use with the present inventioninclude sodium dodecyl sulfate (SDS) and aerosol OT (AOT). The preferredsize of reverse micelles for use with the present invention are of adiameter from about 20 nanometers to about 60 nanometers, with about 40nanometers being particularly preferred.

FIG. 4 is a process flow diagram for one embodiment of the presentinvention involving use of a microemulsion system to prepare nano-sizemagnetic particles. As shown in FIG. 4, a metal-containing firstreactant 120 is placed into the disperse aqueous phase of amicroemulsion 122. A second reactant 124 is added to the microemulsion122 to cause a reaction 126 involving the first reactant 120 and thesecond reactant 124 to form metal-containing precursor particles 128.The precursor particles 128 are then subjected to controlled oxidation130 in the presence of an oxidant 132 and magnetic particles 134 areformed.

To prepare iron-containing magnetic particles 134, the first reactant120 is preferably an iron-containing salt dissolved in aqueous dropletsof the disperse phase of the microemulsion 122. Iron-containing magneticparticles 134 include those of magnetite and maghemite. The salt may beany suitable salt, including a nitrate, sulfate or halide salt, andpreferably is a ferrous salt. A particularly preferred iron-containingsalt is ferrous chloride, which may be in the form of a hydrate.

The microemulsion 122 may be prepared by mixing an aqueous liquid havingthe first reactant in solution with a suitable surfactant and a nonpolarorganic liquid. The mixture is then vigorously mixed to convert theaqueous liquid into disperse phase droplets in a microemulsion. Thedisperse phase preferably comprises droplets having a size of smallerthan about 60 nanometers. Preferably, the process is conducted under ablanket of inert gas, such as nitrogen gas, beginning with preparationof the microemulsion 122 and continuing through the controlled oxidation130.

A preferred surfactant is SDS, which is more preferably used incombination with AOT as a cosurfactant. Preferably, the amount ofsurfactant used is in the range of 30 to 70 weight percent relative towater for SDS and in the range of 250 to 400 weight percent relative towater for AOT.

The organic liquid may comprise any suitable nonpolar organic liquid. Apreferred organic liquid is isooctane because it is relativelyinexpensive and easy to work with. Other possible organic liquidsinclude other alkanes, including normal alkanes, branched alkanes andcycloalkanes (e.g., hexane, heptane, octane, decane, cyclohexane);aromatics including toluene and benzene; and halogenated alkanes such asmethylene chloride. The organic liquid is generally present in an amountgreater than about 30 parts organic liquid to one part of the aqueousliquid.

The second reactant 124 may be any material capable of reacting with thefirst reactant 120 to assist in the formation of the desired precursorparticles 128. The first reactant 120 is typically dissolved in thedisperse aqueous phase of the microemulsion 122 and the reaction 126causes precipitation in the disperse phase to form the desired precursorparticles 128. The disperse aqueous phase typically has an acidic pH offrom about 3 to about 5 prior to the reaction 126. The second reactant124 generally comprises a basic material. The pH of the disperse aqueousphase, therefore, increases during the reaction 126. The second reactantcould be a hydroxide such as an alkali, ammonium or alkaline earthhydroxide. Such hydroxides would typically be provided in a separatemicroemulsion having the hydroxide dissolved in a disperse aqueousphase. The microemulsion having the second reactant 124 could then beadded to the microemulsion 122 to cause the reaction 126.

Although hydroxides may be used as the second reactant 124, asdescribed, it is preferred that less basic materials be used. The use ofa hydroxide as the second reactant 124 generally requires that a bufferalso be added to prevent the pH in the disperse aqueous phase of themicroemulsion 122 from becoming too high. The pH of the aqueous dispersephase of the microemulsion 122 is extremely important to ensure ultimateproduction of the desired magnetic particles 134. If the magneticparticles 134 are of magnetite, the pH in the disperse aqueous phaseshould be raised to an ending pH of from about pH 8 to about pH 10. Ahigher pH will tend to result in the ultimate preparation of nonmagnetichematite (alpha-Fe₂ O₃) rather than the desired magnetite. If themagnetic particles are to be of maghemite, then the pH in the disperseaqueous phase should raised to an ending pH of no higher than about pH7, and preferably to a pH of from about pH 6 to about pH 7. Preferred asthe second reactant 124 are ammonia and amines, with amines being morepreferred. Alkyl amines, such as triethylamine, are particularlypreferred.

Furthermore, it is preferred that the second reactant 124 be soluble inboth the continuous phase and the disperse phase of the microemulsion122. It would, therefore, be unnecessary to prepare a separatemicroemulsion for the purpose of adding the second reactant 124 to themicroemulsion 122. Rather, the second reactant 124 may be dissolved in aseparate batch of the organic liquid and added to the continuous phaseof the microemulsion 122. Alkyl amines such as triethylamine have suchdual solubility.

When making iron-containing magnetic particles 134, the precursorparticles will typically be a green rust. The green rust precursorparticles comprise a complex involving ferrous hydroxide. In thecontrolled oxidation 130, the green rust is oxidized to permit formationof the desired magnetic iron oxide for the magnetic particles 134.

The controlled oxidation 130 is preferably accomplished while theprecursor particles 128 remain dispersed in the disperse aqueous phaseof the microemulsion 122. The microemulsion 122 may be heated toaccelerate the controlled oxidation 130.

The oxidant 132 may be any oxygen-containing material capable ofoxidizing the precursor particles. It is essential that the rate atwhich oxidation occurs, however, be carefully controlled to avoidpreparation of nonmagnetic materials. When making magnetite ormaghemite, if oxidation is not carefully controlled, a nonmagnetichematite product will be readily produced rather than the desiredmagnetic material. It is preferred that the controlled oxidation 130 beconducted substantially in the absence of gaseous oxidants, andespecially in the absence of free oxygen gas, because it is difficult tocontrol the oxidation in the presence of such gaseous oxidants. Rather,it is preferred that oxidation be controlled by providing an oxidant 132from a liquid solution and that a carefully controlled amount of theoxidant 132 be used. For example, when isooctane is used as a continuousorganic phase, the isooctane may be saturated with oxygen by aerationprior to preparation of the microemulsion 122. The oxygen in solution inthe isooctane then supplies the necessary oxidant in the controlledoxidation 130 for preparation of the magnetic particles 134.

A more preferred oxidant is to use an oxygen-containing salt dissolvedin an aqueous phase liquid that may be added to the microemulsion 122having the precursor particles 128. Preferred salts are those of alkalimetals, especially sodium and potassium nitrates and nitrites. Forexample, sodium nitrite may be dissolved in water and the water used toform a disperse aqueous phase in a separate microemulsion that is addedto the microemulsion 122 to provide the oxidant 132. The sodium nitriteprovides the necessary oxygen for oxidation of the precursor particles128 to form the desired magnetic particles 134. Additional oxidants thatmay be used include peroxides, and especially hydrogen peroxide. Asnoted previously, the process should preferably be conducted under anatmosphere of inert gas, such as nitrogen gas, until the magneticparticles 134 have been formed.

The size of magnetic particles 134 produced depends mainly upon theamount of surfactant used relative to water in preparing themicroemulsion 122. A larger relative amount of surfactant generallyleads to larger dispersed aqueous droplets and, accordingly, to largerresulting magnetic particles 134. The size of the magnetic particles 134may be varied from about 1 nanometer to about 100 nanometers. Preferredmagnetic particles 134 have at least about 90 weight percent of theparticles being of smaller than about 60 nanometers in size and morepreferably smaller than about 40 nanometers in size. Magnetic particles134 having at least 90 weight percent of the particles between about 10nanometers and about 40 nanometers are particularly preferred.

The process of the present invention permits manufacture of magneticparticles 134 having a very narrow size distribution with preferably 90weight percent of the magnetic particles 134 being from about 50 percentsmaller to about 50 percent larger than the average particle size, andmore preferably from about 20 percent smaller to about 20 percent largerthan average particle size. The average particle size may be obtainedthrough transmission electron microscopy electronically from generatedimages through available software and manually by measuring thediameters of particles in a representative sampling and computing asimple average of the particle diameters.

The magnetic particles 134 also have excellent magnetic properties foruse in reprographic processes. Although the saturation magnetism islower than for larger, conventional magnetic particles, saturationmagnetism remains satisfactory in the magnetic particles 134 of thepresent invention. Magnetic particles 134 of smaller than about 40nanometers have a saturation magnetism that is preferably greater thanabout 15 emu/g, more preferably greater than about 25 emu/g and mostpreferably greater than about 35 emu/g. The high saturation magnetism ofthe magnetic particles 134 is believed to be due to the high purity ofthe particles produced. For magnetite and maghemite, particles in excessof 90 weight percent in purity of the magnetic iron oxide are obtained,with purities often exceeding 95 weight percent.

After the magnetic particles 134 have been formed, the magneticparticles 134 may be recovered by breaking the microemulsion. Themicroemulsion may be broken using any suitable technique. One effectivetechnique is to simply add sufficient water to cause the aqueous phasedroplets to coalesce, with the magnetic particles 134 staying with theaqueous phase. The organic liquid may be decanted off and the magneticparticles recovered from the remaining aqueous liquid by any suitablemethod such as by filtration or centrifugation.

For additional information concerning preparation of magnetic particlesinvolving precipitation in a disperse aqueous phase of a microemulsionreference is made to Selim et al., "Preparation of Nano-size MagneticGamma-Ferric Oxide (gamma-Fe₂ O₃) and Magnetite (Fe₃ O₄) Particles forToner and Color Imaging Applications", Proceedings of IS&T 11thInternational Congress on Advances on Nonimpact Printing Technologies,1995, pp. 106-109; and a thesis by Lyle P. Cunningham entitled"Preparation of Nanocrystalline Magnetic Maghemite (gamma-Fe₂ O₃) andMagnetite (Fe₃ O₄) Particles in Microemulsions", available at the ArthurLakes Library of the Colorado School of Mines, Golden, Colo., thecontents of both of which are incorporated by reference herein in theirentireties.

One problem with nano-size particles is that they have a tendency toagglomerate. For the nano-size magnetic particles of the presentinvention, agglomeration of the particles would detrimentally affecttheir use in toner products.

In one aspect of the present invention, the nano-size magnetic particlesare subjected to a chemical surface treatment to impart improvedhydrophobicity to the particles and to thereby reduce the tendency ofthe particles to agglomerate and to improve dispersibility of theparticles to aid in the manufacture of toner and other reprographicprint media. The surface treatment preferably occurs prior to a completecentrifugation or other processes that would tend to promoteagglomeration.

In one embodiment of the surface treatment, the particles are treatedwith an amphiphilic material. Preferred amphiphilic materials are saltsof fatty acids, such as sodium or potassium laurate or stearate. In apreferred surface treatment, however, hydrophobic chemical groups areconnected with the nano-size magnetic particles through covalentbonding, preferably with use of a silane coupling agent.

An example of providing the hydrophobic surface treatment throughcovalent bonding of a hydrophobic chemical group is as follows.Referring back to the discussion concerning the manufacture of nano-sizemagnetic particles as described with reference to FIG. 1, after breakingof the microemulsion, the nano-size magnetic particles are permitted tosettle and the organic liquid is decanted. After the magnetic particlessettle, the aqueous liquid is decanted. The magnetic particles are thenwashed to remove surfactants and residual organic and inorganicmaterials that may be contaminating the magnetic particles. The wash mayinclude any suitable solvent. One solvent that has been used is amixture of chloroform and water. The magnetic particles are typicallywashed three times with partial centrifugation followed by decantationof the wash liquid between washings. Partial centrifugation refers tocentrifugal processing in which centrifugal separation does not proceedto completion. Centrifuging is terminated before the liquid volume isreduced to a point where significant interaction between the magneticparticles occurs. Complete centrifugation should be avoided at thisstage because such processing could promote undesirable agglomeration ofthe magnetic particles.

After washing is complete, an aqueous acidic solution is added to themagnetic particles to form a slurry having about 10 weight percent ofsolids. The acidic aqueous solution is preferably about pH 4 andpreferably comprises an aqueous solution of glacial acetic acid. Whilethe magnetic particles are suspended in the aqueous acidic solution, asolution comprising a silane-based compound to act as a silane couplingagent is added to the solution and the solution heated to cause reactionbetween the magnetic particles and silane coupling agent. The amount ofthe silane-based material is preferably from about 0.5 to about 5 weightpercent of the magnetic particles. Preferably, the slurry is sonicatedduring the procedure to maintain the magnetic particles in a dispersesuspension. After adding the silane-based compound the slurry is heated,preferably to about 50° C., to promote reaction between the silane-basedcompound and the magnetic particles. Following reaction with thesilane-based compound, the magnetic particles may be separated from theslurry by a complete centrifugation to recover the hydrophobicallytreated magnetic particles.

As used herein, a silane-based compound includes all compounds includinga silicon atom covalently bonded to four constituent chemical groups.Preferred silane-based compounds include alkoxy silanes. One example ofsuch an alkoxy silane is vinyltriacetoxysilane, in which the vinyl groupacts as the hydrophobic chemical group to reduce the tendency foragglomeration of the magnetic particles. Other hydrophobic groupsinclude other alkyl groups (saturated and unsaturated, branched andnormal), cycloalkyl groups and aromatic groups.

The silane-based compound reacts with hydroxyl groups at the surface ofthe magnetic particles to form a covalent bond. For example, surfacetreatment involving vinyltriacetoxysilane would proceed according to thefollowing chemical equation:

    (magnetic particle)--OH+CH.sub.2 CHSi(OCOCH.sub.3).sub.3 →(magnetic particle)--O--Si(OCOCH.sub.3).sub.2 CHCH.sub.2 +CH.sub.3 COOH

In another embodiment, the hydrophobic chemical group at least partiallymasks the inherent color of the magnetic particle. Because of thismasking, the inherent color of the magnetic particles does not asgreatly distort the color of dyes or pigments mixed in a toner or otherprinting medium product. Preferably, the hydrophobic chemical groupcomprises a chromophore to provide a desired colorant and that alsomasks, at least in part, the inherent color of the magnetic particles.

When hydrophobic surface treatment involves the use of a hydrophobicchemical group comprising a chromophore, the treatment generallyproceeds in two steps. In a first step, a silane-based compound, to actas a silane coupling agent, is reacted with the magnetic particles asdescribed previously. The silane-based compound in this embodiment,however, also has a reactive group that may react, in a second step,with a reactive dye containing the desired chromophore. The reactivegroup on the silane-based compound may be a hydroxyl group, amino groupor thiol group, with an amino group being preferred. Examples ofsuitable silane-based materials useful as silane coupling agents forthis embodiment of the invention include 3-aminopropyltriethoxysilaneand 3-aminopropyltrimethoxysilane. The reactive dye is a compound havinga chromophore linked to a functional group capable of reacting with thehydroxyl, amino or thiol group of the silane-based compound. As usedherein, a silane coupling agent includes not only the unreactedsilane-based compound but also the residual chemical group followingreaction of the silane-based compound with the magnetic particle and/orwith the reactive colorant.

Table 1 shows some reactive dyes useful with the present invention.These dyes are obtainable from Imperial Chemical Industries. As anexample of attaching C.I. Reactive Blue 2 to a nano-size magnetiteparticle, a silane coupling agent, such as 3-aminopropyltriethoxysilanewould first be reacted with the magnetic particle as shown in Equation Iin FIG. 5. After reaction with the silane coupling agent, the treatedparticle would then be reacted with the reactive dye as shown inEquation II in FIG. 5 to form the final colored particle.

                  TABLE 1                                                         ______________________________________                                        Dye           Reactive Group Chromophore                                      ______________________________________                                        Procion Turquoise HA                                                                        Monochlorotriazine                                                                           Phthalocyanine                                   C.I. Reactive Blue 71                                                         Procion Yellow MX-8G                                                                        Dichlorotriazine                                                                             Azo                                              C.I. Reactive Yellow 86                                                       Levafix Brilliant Red E-                                                                    Dichloroquinoxaline                                                                          Azo                                              6BA                                                                           C.I. Reactive Red 159                                                         C.I. Reactive Blue 2                                                                        Monochlorotriazine                                                                           Anthraquinone                                    ______________________________________                                    

EXAMPLES

The following examples further demonstrate the present invention withoutlimiting the scope thereof.

Example 1

This example demonstrates preparation of nano-size magnetic particlescomprising magnetite using a microemulsion system. A microemulsionconsisting of isooctane as the continuous phase, aqueous ferrouschloride as the dispersed phase, sodium dodecyl sulfate (SDS) as thesurfactant, and aerosol *OT (AOT) as co-surfactant is made according tothe following procedure. Accurately weigh 1.72 g SDS and 11.61 g AOT andplace them in a 500 ml Erlenmeyer flask. To this solid mixture, add 172ml of isooctane. Next, add 4 ml of 1.0M ferrous chloride (FeCl₂.4H₂ O)solution. The mixture is then stirred vigorously with a magnetic stirrerfor about 4 hours under nitrogen atmosphere until a clear pale yellowmicroemulsion is formed.

A second microemulsion is prepared by accurately weighing 1.72 g SDS and11.61 g AOT and placing them in a 500 ml Erlenmeyer flask. To this solidmixture, add 172 ml of isooctane. Next, add 4 ml of 2.0M sodium nitrite(NaNO₂) solution. The mixture is then stirred vigorously with a magneticstirrer for about 4 hours under nitrogen atmosphere until a clearcolorless microemulsion is formed.

Under nitrogen atmosphere, add 8 ml of triethylamine (TEA) to theferrous chloride microemulsion. A bluish green precipitate isimmediately formed upon addition of the TEA solution indicating theformation of green rust precursor particles. To this add 15 ml of thesodium nitrite microemulsion and heat the suspension to 50° C. for 10minutes while stirring. The green rust transforms through oxidation to ablack solution/slurry of magnetite particles suspended in the reversemicelles of the microemulsion. A representative x-ray diffractionpattern for the magnetite is shown in FIG. 6 and demonstrates that theparticles comprise about 98% of magnetite.

Example 2

This example demonstrates preparation of maghemite particles using amicroemulsion system. A microemulsion consisting of isooctane as thecontinuous phase, aqueous ferrous chloride as the dispersed phase,sodium dodecyl sulfate (SDS) as the surfactant, and aerosol *OT (AOT) asco-surfactant was made according to the following procedure. Accuratelyweigh 1.72 g SDS and 11.61 g AOT and place them in a 500 ml Erlenmeyerflask. To this solid mixture, add 172 ml of isooctane. Next, add 4 ml of1.0M ferrous chloride FeCl₂.4H₂ O solution. The mixture is then stirredvigorously with a magnetic stirrer for about 4 hours under nitrogenatmosphere until a clear pale yellow microemulsion is formed.

A second microemulsion is prepared by accurately weighing 1.72 g SDS and11.61 g AOT and placing them in a 500 ml Erlenmeyer flask. To this solidmixture, add 172 ml of isooctane. Next, add 4 ml of 2.0M sodium nitriteNaNO₂ solution. The mixture is then stirred vigorously with a magneticstirrer for about 4 hours under nitrogen atmosphere until a clearcolorless microemulsion is formed.

Under nitrogen atmosphere, add 2 ml of triethylamine (TEA) to theferrous chloride microemulsion. A bluish green precipitate isimmediately formed upon addition of the TEA solution indicating theformation of green rust precursor particles. To this add 30 ml of thesodium nitrite microemulsion and heat the suspension to 50° C. for 10minutes while stirring. The green rust transforms through oxidation to adark brown solution/slurry of maghemite particles suspended in thereverse micelles of the microemulsion. A representative x-raydiffraction pattern for the maghemite particles is shown in FIG. 7 andshow that the particles comprise about 98% of maghemite.

Example 3

This example demonstrates hydrophobic treatment of magnetite particlesprepared as described in Example 1. According to the procedure ofExample 1, after oxidation of the green rust to form the magnetiteparticles, sufficient water is added to the microemulsion system tobreak the emulsion. The organic liquid is decanted and the magnetiteparticles are permitted to settle in the remaining aqueous liquid. Theaqueous phase liquid is decanted and the magnetite particles are washedthree times with a mixture of water and chloroform to remove residualsurfactant, organic and inorganic contaminants, with partialcentrifugation between washings.

The washed magnetite particles are then slurried with an acidic aqueoussolution of glacial acetic acid (pH 4.0) to form a slurry with tenweight percent of the magnetite particles. The magnetite particles aremaintained in suspension by sonication while two drops of liquidvinyltriacetoxysilane are added to the slurry. The slurry is then heatedto 50° C. for 30 minutes. The slurry is then centrifuged to obtain dryparticles.

FIG. 8 shows a transmission electron micrograph of representativemagnetite particles hydrophobically treated with vinylacetoxysilane. Asseen in FIG. 8, the particles have an extremely narrow size distributionand are approximately 20 nanometers in size. Table 2 shows the magneticproperties of the hydrophobically treated nano-size magnetite particlescompared to commercially available magnetite of about 0.5 micron sizeobtainable from Magnox Incorporated. As shown in Table 2, although thenano-size magnetite particles are extremely small, they still possesssufficient saturation magnetism to permit them to be used as magneticparticles in toner and other print medium products.

                  TABLE 2                                                         ______________________________________                                                               Saturation                                                        Saturation  (Magnetism                                             Materials  Magnetism (Mx)                                                                            (emu/g)    Coercivity (Oe)                             ______________________________________                                        Prepared Fe.sub.3 O.sub.4                                                                218.368     38.4       24.955                                      HARCROS TB 548.933     88.9       98.525                                      5600                                                                          STANDARD I                                                                    MAGNOX TMB 588.178     85.7       53.49                                       114                                                                           STANDARD II                                                                   ______________________________________                                    

Example 4

This example demonstrates preparation of magnetite particles in whichthe oxidant is dissolved in the continuous phase of the microemulsion.In a 250 ml Erlenmeyer flask is placed one gram of SDS and 6.75 gramsAOT. Aerated isooctane is supplied by subjecting isooctane to spargingwith air to saturate the isooctane with oxygen. To the Erlenmeyer flaskis added 100 ml of the aerated isooctane and the mixture is vigorouslystirred with a magnetic stirrer. Next is added 2 ml of 1.0M FeCl₂.4H₂ Osolution and stirring is continued for about two hours under nitrogenatmosphere. A clear microemulsion with a pale yellow color is formed.

A second solution is prepared by dissolving 0.92 gm of triethylamine(TEA) and 100 ml of the aerated isooctane. Under nitrogen atmosphere,the TEA-isooctane solution is added to the microemulsion and theErlenmeyer flask is sealed. A bluish green precipitate is immediatelyformed upon addition of the TEA solution to the ferrous chloridemicroemulsion, indicating the formation of green rust precursorparticles. The suspension is next heated under the nitrogen atmosphereto 50° C. for one hour while stirring. The green rust transforms throughslow oxidation to a black solution/slurry of magnetite particlessuspended in the reverse micelles of the microemulsion system.

The magnetic particles of the present invention are useful in toner andother print media products used in reprographic processing. Toner isgenerally made by mixing the magnetic particles with a polymer resin.The mixing may occur in a polymer melt or may be accomplished by in-situpolymerization in the presence of magnetic particles mixed withappropriate monomer. As is conventionally known, additional componentsmay be added to toner compositions such as colorants, including pigmentsand dyes; charge control agents, such as organometallics; and othermodifying agents, such as waxes to assist cleaning of equipment in whichthe toner is used or other materials to increase friability of thetoner.

Polymer resins useful for the present invention are those conventionallyknown for use in toner compositions. Such polymer resins may includerepeating units from polymerization of one or more of the followingmonomers: styrene, and derivatives thereof such as styrene,o-methylstyrene, m-methylstyrene, p-methylstyrene, p-methoxystyrene,p-phenylstyrene, p-chlorostyrene, 2,4-dichlorostyrene, p-ethylstyrene,2,4-dimethylstyrene, p-n-butylstyrene, p-tertbutylstyrene,p-n-hexylstyrene, p-n-octylstyrene, p-n-nonylstyrene, p-n-decylstyrene,p-n-dodecylstyrene and the like; unsaturated monoolefins such asethylene, propylene, butylene, isobutylene and the like; vinyl halidessuch as vinyl chloride, vinylidene chloride, vinyl bromide, vinylfluoride and the like; vinyl esters such as vinyl acetate, vinylpropionate, vinyl benzoate and the like; α-methylene aliphaticmono-carboxylic acid esters such as methyl methacrylate, ethylmethacrylate, propyl methacrylate, n-butyl methacrylate, isobutylmethacrylatic, n-octyl methacrylate, dodecyl methacrylate, 2-ethylhexylmethacrylate, stearyl methacrylate, phenyl methacrylate,dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate and thelike; acrylic acid esters such as methyl acrylate, ethyl acrylate,n-butyl acrylate, isobutyl acrylate, propyl acrylate, n-octyl acrylate,dodecyl acrylate, 2-ethylhexyl acrylate, stearyl acrylate, 2-chloroethylacrylate, phenyl acrylate and the like; vinyl ethers such as vinylmethyl ether, vinyl ethyl ether, vinyl isobutyl ether and the like;vinyl ketones such as vinyl methyl ketone, vinyl hexyl ketone, vinylisopropenyl ketone and the like; N-vinyl compounds such asn-vinylpyrrole, n-vinylcarbazone, N-vinylindone, N-vinylpyrrolidone andthe like; vinyl-naphthalenes; acrylic acid or methacrylic acidderivatives such as acrylonitrile, methacrylonitrile, acrylamide, andothers.

Toner is typically particulized as small particles. Preferably, thetoner particles are smaller than about 10 microns, more preferablysmaller than about 8 microns, and even more preferably smaller thanabout 6 microns. The toner may be made as a particulate by an extrusionprocess of the polymer resin/magnetite mixture or through comminutionprocessing. Alternatively, the toner may be made in small particles byemulsion or suspension polymerization of the polymer resin in thepresence of the magnetic particles to form in-situ the particles of thedesired size.

The toners of the present invention are useful in reprographicprocessing, as previously discussed. Two major types of reprographicprocessing for which the materials of the present invention are wellsuited include electrophotographic processing and magnetographicprocessing, the fundamentals of both of which are well known in the art.In electrophotographic processes, an image is developed with toner on aphotoconductive drum and the toner image is then transferred to a sheetsubstrate, such as paper. In a magnetographic process, the desired tonerimage is developed on a magnetic drum and the toner image is thereaftertransferred to a sheet substrate such as paper. Also, the toner productsof the present invention are useful for producing either spot color(e.g., highlight color) or process color, in which primary colors aremixed as needed to give a total color gambit.

It should be noted that the toner products of the present inventionprovide improved coloration properties due to the extremely small sizeof the nano-size magnetic particles and the surface treatment of thoseparticles to provide hydrophobic chemical groups that at least partiallymask the inherent color of the magnetic particles. Therefore, themagnetic particles are easier to incorporate into a color toner productwithout the same level of dulling of colors as occurs withconventionally available magnetic particles.

Various embodiments of the present invention have been described indetail. It should be recognized that any elements of any of thesedescribed embodiments can be combined in any combination with elementsof any other embodiment. Furthermore, modifications and adaptations ofthe disclosed embodiments will be apparent to those skilled in the art.It is to be expressly understood that such modifications and adaptationsare within the scope of the present invention as set forth in thefollowing claims.

What is claimed is:
 1. A method for making free flowing magnetic nanoparticles useful for reprographic processes, the method comprising the steps of:providing magnetic particles, at least 90 weight percent of which have a size of smaller than about 60 nanometers; chemically treating a surface of said magnetic particles to increase the hydrophobicity and dispersibility and to decrease the agglomeration tendency of said magnetic particles, and thereby enhance the free flowability of said magnetic particles; wherein said chemical treating comprises attaching a hydrophobic chemical group to said magnetic particles through covalent bonding.
 2. The method of claim 1, wherein:said covalent bonding comprises a silane coupling agent intermediate between one of said magnetic particles and said hydrophobic chemical group.
 3. The method of claim 1, wherein:said step of chemically treating a surface of said particles comprises reacting said particles with a silane-based compound having a hydrophobic chemical group.
 4. The method of claim 3, wherein:said silane-based compound comprises vinyltriacetoxysilane.
 5. The method of claim 1, wherein:at least 90 weight percent of said magnetic particles have a size smaller than about 40 nanometers.
 6. The method of claim 1, wherein:greater than about 90 weight percent of said magnetic particles have a size that is from about 50 percent smaller than to about 50 percent larger than a weight median size of said magnetic particles.
 7. The method of claim 1, wherein:said particles have a saturation magnetism of larger than about 25 emu/g.
 8. The method of claim 1, wherein:said step of providing said magnetic particles comprises precipitation, in a disperse phase of a microemulsion, of metal-containing precursor particles and converting said metal-containing precursor particles to said magnetic particles; and wherein said step of chemically treating a surface of said magnetic particles is performed after said step of converting and before said magnetic particles are dried, to prevent said magnetic particles from agglomerating prior to said step of chemically treating.
 9. The method of claim 1, wherein:said step of providing said magnetic particles comprises precipitation of metal-containing precursor particles in a disperse phase of a microemulsion; and converting said metal-containing precursor particles to said magnetic particles.
 10. The method of claim 1, wherein:said magnetic particles comprise greater than about 90 weight percent of at least one magnetic iron oxide selected from the group consisting of magnetite (Fe₃ O₄), maghemite (gamma-Fe₂ O₃) and combinations thereof.
 11. A reprographic print medium, comprising:toner particles including iron-containing magnetic particles and a polymer, at least 90 weight percent of said magnetic particles being smaller than about 60 nanometers in size; wherein a surface of said magnetic particles is at least partially covered by a surface coating material having hydrophobic properties, to increase the ease with which said magnetic particles may be dispersed and to reduce the tendency of said magnetic particles to agglomerate; said surface coating material comprises a hydrophobic chemical group attached to a surface portion of said magnetic particles through covalent bonding.
 12. The reprographic print medium of claim 11, wherein:said hydrophobic chemical group is attached to a surface portion of said particles by covalent bonding through a silane coupling agent.
 13. The reprographic print medium of claim 11, wherein:said surface coating material comprises the reaction residue of vinyltriacetoxysilane that is covalently bonded with a surface portion of said magnetic particles.
 14. The reprographic print medium of claim 11, wherein:said magnetic particles result from a process of manufacture comprising precipitation of a metal-containing particle in a disperse phase of a microemulsion.
 15. The reprographic print medium of claim 11, wherein:said surface coating material comprises a hydrophobic chemical group that masks, at least in part, a natural color of said magnetic particles.
 16. The reprographic print medium of claim 11, wherein:said surface coating material comprises a chromophore attached with said magnetic particles through covalent bonding.
 17. Magnetic particles with improved dispersibility useful in making toner for use in reprographic processes, the magnetic particles each comprising:an iron-containing magnetic core having a size of smaller than about 60 nanometers; hydrophobic chemical groups about the outer surface of the iron-containing magnetic core to increase dispersibility of said core and to decrease the susceptibility of said core to agglomerate with other such cores, said hydrophobic chemical groups attached with said iron-containing magnetic cores through covalent bonding.
 18. The magnetic particles of claim 17, wherein:said iron-containing magnetic core results from a process of manufacture comprising precipitation of iron-containing particles in a disperse phase of a microemulsion.
 19. The magnetic particles of claim 17, wherein:said hydrophobic chemical groups are attached to said outer surface of said iron-containing magnetic core through a silane coupling agent.
 20. The magnetic particles of claim 19, wherein:said hydrophobic chemical groups at least partially mask a natural color of said iron-containing magnetic core.
 21. The magnetic particles of claim 19, wherein:said hydrophobic chemical groups comprise a chromophore.
 22. The magnetic particles of claim 17, wherein:said iron-containing magnetic core has a size of smaller than about 40 nanometers.
 23. The magnetic particles of claim 17, wherein:said magnetic particles have a saturation magnetism of larger than about 25 emu/g.
 24. The magnetic particles of claim 17, wherein:said iron-containing magnetic core comprises at least one of magnetite (Fe₃ O₄) and maghemite (gamma-Fe₂ O₃).
 25. The reprographic print medium of claim 11, wherein:said toner particles are of a size that is smaller than about 10 microns. 